1. Field of the Invention
This invention relates in general to capping or blocking a pipe end and, in particular, to capping of a broken subsea riser.
2. Brief Description of Related Art
In subsea drilling operations, drilling operators generally deploy remotely operated vehicles (ROVs) to the wellhead in emergency situations to enable devices designed to cap, cut off, or contain the flow of hydrocarbons from a well. In some instances, a remotely operated vehicle will activate a blowout preventer (BOP) designed to shut off the flow of hydrocarbons from the wellhead. Activating a BOP will engage rams within the BOP that pinch shut or otherwise disable the wellhead in a manner that significantly limits the ability of the operators to continue use of the wellhead. Therefore, there is a need for an apparatus to cap, cut off, or contain the flow of hydrocarbons from a wellhead without limiting the ability of the operators to continue to use the wellhead.
A second way drilling operators attempt to contain flow of hydrocarbons from a wellhead in emergency situations involves a containment dome or “Top Hat”. Use of a containment dome involves lowering a large device over the wellhead to contain flowing hydrocarbons. Oil workers attach riser pipes to the containment dome to remove the hydrocarbons collected within the containment dome. In this manner, the containment dome captures hydrocarbons from a wellhead for transportation to surface vessels. However, use at the depths of some deepwater drilling sites causes methane hydrate crystals to form within the containment dome. These methane hydrate crystals block the openings that oil workers use to remove hydrocarbons from the containment dome preventing capture of the hydrocarbons.
Operators may simply attempt to place a cap having a sufficient weight to overcome the pressure of the wellbore fluids on top of the wellhead. However, in many situations the wellbore riser does not have a suitable surface for the cap, and the wellbore fluids may flow at too great of a pressure to be overcome by the weight of the cap. In some instances, operators may attempt to weld a flange over the pipe end to block the pipe passageway. However, due to the operating conditions at many subsea wellheads, and the pressures of the wellbore fluids, welding a flange to the pipe end is often not possible. Therefore, there is a need for an apparatus to aid in the blockage or capture of hydrocarbons from a wellhead located at great depth without relying on weight or an operators ability to weld subsea.
Oil operators sometimes engage a method called “top kill” to cap or cut off the flow of hydrocarbons from a wellhead in emergency situations. In this procedure, oil workers connect drilling pipe to the BOP through a manifold. Oil workers then pump drilling mud into the well in sufficient quantities to slow and then stop the passage of hydrocarbons from the wellhead. Once the drilling mud reaches sufficient quantities to overcome the reservoir pressure at the wellhead, hydrocarbon flow stops, and oil workers use cement to seal the well. In instances where drilling mud alone is insufficient to stop hydrocarbon flow, oil workers will utilize a “junk shot”. A junk shot involves pumping materials of a more solid nature along with more drilling mud into the wellhead in an effort to block or plug the flow of hydrocarbons. Much like use of a BOP, top kill and junk shots effectively stop any further use of the wellhead for the production of hydrocarbons. In addition, many times junk shots are ineffective, failing to stop flow of fluids from the wellhead. Therefore, there is a need for an apparatus that can stop hydrocarbon flow from a wellhead without limiting further use of the well or relying on ineffective junk shots.
Another method operators use to contain the flow of hydrocarbons from a wellhead in emergency situations involves cutting off the end of a lower riser and capping the wellhead with a modified Lower Marine Riser Package (LMRP). This method, similar to the containment dome, attempts to direct the flow of hydrocarbons into a subsea containment vessel from which oil workers pump the hydrocarbons for further action. Unlike the containment dome, LMRP does not attempt to collect and contain all the hydrocarbons from the wellhead. Thus, even where used, all hydrocarbon flow is not stopped or contained. LMRP also makes complete capping of the well more difficult by shearing off the riser line. Shearing off the riser line removes any blockages from the hydrocarbon path that slowed the rate of hydrocarbon flow, thus making it more difficult to eventually cap or contain the well completely. At times, shearing off the end of a lower riser is necessary to perform other operations at the wellhead. Thus, there is a need for an apparatus that can cap, cut off, or contain the flow of hydrocarbons where a riser has been sheared off for other purposes.